The present invention relates to a port for a fan chamber, and more specifically to combustion-powered fastener driving tools utilizing a fan chamber having a port through which pressured gases from a chamber volume are expelled to an outside volume of lower pressure.
Gas combustion devices are known in the art. A practical application of this technology is found in combustion-powered fastener driving tools. One type of such tools, also known as IMPULSE® brand tools for use in driving fasteners into workpieces, is described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,522,162, 4,483,473, 4,483,474, 4,403,722, 5,197,646 and 5,263,439, all of which are incorporated by reference herein. Similar combustion powered nail and staple driving tools are available commercially from ITW-Paslode of Vernon Hills, Ill. under the IMPULSE® brand, and from ITW-S.P.I.T. of Bourg-les-Valence, France under the PULSA® brand.
A known combustion-powered fastener driving tool is shown in FIG. 1. The tool 10 incorporates a generally pistol-shaped tool housing 12 enclosing a small internal combustion engine 14. The engine is powered by a canister of pressurized fuel gas (not shown), also called a fuel cell. A battery-powered electronic power distribution unit (not shown) produces a spark for ignition, and a fan 16 located in a combustion chamber 18 provides for both an efficient combustion within the chamber 18, while facilitating processes ancillary to the combustion operation of the device.
Such ancillary processes include: inserting fuel into the chamber 18; mixing the fuel and air within the chamber 18; and removing, or scavenging, combustion by-products. In addition to these ancillary processes, the fan further serves to cool the tool and increase combustion energy output. The engine 14 includes a reciprocating piston 20 with an elongated, rigid driver blade 22 disposed within a single cylinder body 24.
A valve sleeve 26 is axially reciprocable about the cylinder body 24 and, through a linkage (not shown), moves to close the combustion chamber 14 when a work contact element 28 at the end of the linkage is pressed against a workpiece 30. This pressing action also triggers a fuel-metering valve (not shown) to introduce a specified volume of fuel into the closed combustion chamber 18.
Upon the pulling of a trigger switch 32, which causes a spark to ignite a charge of gas in the combustion chamber 18 of the engine 14, the piston 20 and driver blade 22 are shot downward to impact a positioned fastener (not shown) and drive the fastener into the workpiece 30. The piston 20 then returns to its original, or “ready” position, through differential gas pressures within the cylinder body 24. Fasteners are fed magazine-style into a nosepiece 34, where the fasteners are held in a properly positioned orientation for receiving the impact of the driver blade 22.
Upon ignition of the combustible fuel/air gas mixture, the combustion in the chamber 18 transfers the ignited gas through a port 36 in the chamber 18, which causes the acceleration of the piston/driver blade assembly 20/22 and the penetration of the fastener into the workpiece 30, if the fastener is present in the nosepiece 34. Combustion pressure in the chamber 18 is an important consideration because the pressure affects the amount of force with which the piston 20 may drive the fastener. Other important considerations are the amount of time required to drive the piston by the ignited gas sent through the port 36, and to complete the ancillary processes between combustion cycles of the engine.
During combustion, a significant amount of gas needs to be transferred from the combustion chamber 18 to the cylinder body 24 within a short time. The fan 16 accelerates this process by its rotation. Efficiency of the fan 16 is significantly affected by the way the chamber 18 and the cylinder body 24 are designed and connected. The fan 16 also serves several other functions. The fan 16 mixes air and fuel, purges exhaust gases, cools the tool 10, and also increases combustion energy output.
Referring now to FIGS. 2 through 4, the effects of the rotation of the fan 16 in the chamber 18 is illustrated. As the fan 16 rotates, a swirl is generated in the chamber 18 in the direction A. The speed of the swirl is equal to zero at the center of rotation, and is maximized nearest an outer wall 38 of the chamber 18. The chamber 18 is typically shaped as a cylinder to maximize the efficiency of the swirl in a circular direction. The change in the speed distribution of the swirl, from the center of rotation to the outer wall 38, generally can be considered to increase linearly, and as a function of a radius of a circular cross-section of the cylindrical wall 38 of the chamber 18, as best seen in FIG. 2.
The swirling flow is transferred out of the chamber 18 through the port 36, located on a disk-shaped top plate 40 joining the outer wall 38. The plate 40 has at least one port. In order to quickly transfer a significant amount of gas through the port 36, the port is made as large as possible, and positioned away from the center of the plate 40, and toward the outer wall 38, where the speed of the swirl is greatest. The central region of the plate 40 is often solid, and can serve as a convenient location for mounting a valve and limiter combination 42, as best seen in FIG. 3 shown in relief.
The shape of the port 36 and valve/limiter 42 determines the resulting cross-sectional flow distribution of gas through the port 36. The present inventor has discovered that, when the port 36 is circular, the resulting cross-sectional flow is elliptical, and inconsistent across and through the port 36, as best seen in FIG. 4. This inconsistency can make the flow of gas through the port 36 unstable, and can thus quench the flame of an ignited gas traveling through the port. Although this quenching is undesirable, it may still be possible to transfer sufficient pressure from the ignited gas in the chamber 18 to fully accelerate the driver blade 22, even if the gas contacting the piston 20 is no longer ignited. Higher energy combustion, however, can be realized by avoiding flame quenching.
There are several significant disadvantages to using a circular port in combustion chambers of this type. A circular port fails to utilize the natural speed distribution of the flow generated by the fan 16. As discussed above, the speed distribution of the flow is considered to increase linearly away from the center of rotation. A circular port will by definition, however, always have one half of its area decreasing away from the center of rotation. Accordingly, the linear area of the circular port 36 farthest away from the center of rotation approaches zero where the speed distribution of the flow is greatest. The circular port 36 therefore fails to allow through it the flow of gas having the greatest energy, which is also an undesirable result.
Multiple combustion chamber systems are utilized in similar tool configurations in order to extract more energy from the combustion. One such preferred system is described in a copending, commonly assigned U.S. Patent Application Ser. No. 10/170,736, which is also incorporated by reference herein. When more than one chamber is used with only one fan, fan efficiency is similarly affected by the way the multiple chambers are designed and connected.
In such a multiple-chamber configuration, the highest output of gas flow through the port is reached by creating a restrictive path for the gas to travel from one chamber to the other. The restrictive path is accomplished by a combination of valves, limiters, ports, and shrouds. The port connecting two chambers typically includes a reed valve, which remains normally closed to prevent back flow of pressure from the second chamber into the first chamber. A limiter physically restricts how far the moving gas may open the valve.
In a tool of this type having a circular port, however, utilizing a more restrictive path to extract more combustion power can in turn negatively affect the tool's ability to transfer gas properly from one chamber to the next during an early stage of combustion. Additionally, when using shrouds, ports, valves, and valve limiters to connect chambers in a multiple-volume combustion chamber with a fan, the operating environment of the system that allows a stable performance becomes significantly narrower. This increased likelihood of instability also increases the likelihood that the flame of the ignited gas passing from one chamber to the next through the restrictive path will be quenched.
Flame quenching in a multiple-chamber configuration can be a significant problem. Pressure build-up from the ignited gas in one chamber can be largely absorbed by the gas in the next chamber when the gas flowing from the first chamber fails to ignite the gas in the next chamber. In other words, gas pressure reaching a piston after the flame is quenched will be less than the pressure from the airflow entering the chamber contacting the piston. The loss in pressure to drive a piston caused by flame quenching becomes even more pronounced as the number of chambers increases.
The instability of the flow through the restrictive path also can decrease the useful lifetime of a valve used in the path. Also, the complexity of the configuration, as well as the number of its required components, both undesirably increase as the airflow path between chambers becomes more restrictive and complex. Accordingly, it is desirable to have an improved configuration which overcomes the above-listed problems.